Biodegradation of MSW

To understand the biodegradation of waste in landfills, it is important to understand the general pathway of anaerobic biodegradation which is a complex process that requires the coordinated activity of several trophic groups of microorganisms (Madigan et al., 2003).

The primary biodegradable constituents in MSW are cellulose (C) and hemicellulose (H), which comprise approximately 40–60% of MSW by dry weight and account for greater than 90% of its methane potential (Barlaz et al. 1990), while the other major organic component, lignin, is at best only slowly degradable under methanogenic conditions (Colberg, 1988). Reported cellulose, hemicellulose, and lignin concentrations in residential refuse range from 28.8 to 54.3%, 6.6 to 11.9%, and 12.1 to 28% of dry weight, respectively (Barlaz, 2006). Data on the cellulose, hemicellulose, and lignin concentrations in municipal waste components are summarized in Table 2.

The first phase is the hydrolysis of polymers (carbohydrates, fats, and proteins), which yields soluble sugars, amino acids, long-chain carboxylic acids, and glycerol. Equation 1. shows an example of a hydrolysis reaction where organic waste is broken down into a simple sugar, in this case, glucose (Ostrem, 2004).

Equation 1 : C6H10O4 + 2H2O → C6H12O6 + 2H2

Fermentation/Acidogenesis

In the second phase, acidogenic bacteria transform the products of the first reaction into short chain volatile acids, ketones, alcohols, hydrogen and carbon dioxide. The principal acidogenesis stage products are propionic acid (CH3CH2COOH), butyric acid (CH3CH2CH2COOH), acetic acid (CH3COOH), formic acid (HCOOH), lactic acid (C3H6O3), ethanol (C2H5OH) and methanol (CH3OH), among other. From these products, the hydrogen, carbon dioxide and acetic acid will skip the third stage, acetogenesis, and be utilized directly by the methanogenic bacteria in the final stage (Figure 9). Equations 2, 3 (Ostrem, 2004) and 4 (Bilitewski et al., 1997) represent three typical acidogenesis reactions where glucose is converted to ethanol, propionate and acetic acid, respectively.

Equation 2 : C6H12O6 ↔ 2CH3CH2OH + 2CO2

Equation 3 : C6H12O6 + 2H2 ↔ 2CH3CH2COOH + 2H2O

Equation 4 : C6H12O6 → 3CH3COOH

Acetogenesis

In the third phase, known as acetogenesis, the rest of the acidogenesis products, i.e. the propionic acid, butyric acid and alcohols are transformed by acetogenic or fatty acid oxidizing bacteria into hydrogen, carbon dioxide and acetic acid (Figure 9). Hydrogen plays an important intermediary role in this process, as the reaction will only occur if the hydrogen partial pressure is low enough to thermodynamically allow the conversion of all the acids. Such lowering of the partial pressure is carried out by hydrogen scavenging bacteria. Equation 5 represents the conversion of propionate to acetate, only achievable at low hydrogen pressure. Glucose (Equation 6) and ethanol (Equation 7) among others are also converted to acetate during the third stage of anaerobic biodegradation (Ostrem, 2004).

Equation 5 : CH3CH2COO-+ 3H2O ↔ CH3COO-+ H++ HCO3-+ 3H2

Equation 6 : C6H12O6+ 2H2O ↔ 2CH3COOH + 2CO2+ 4H2

Equation 7 : CH3CH2OH + 2H2O ↔ CH3COO-+ 2H2+H+

Methanogenesis

The fourth and final phase is called methanogenesis. During this stage, microorganisms convert the hydrogen and acetic acid formed by the acid formers to methane gas and carbon dioxide (Verma, 2002).The most common methanogenic substrates are acetate and CO2 plus H2. The bacteria responsible for this conversion are called methanogens and are strict anaerobes. Most methanogens have a pH optimum around 7 (Zinder, 1993). Should the activity of the fermentative organisms exceed that of the carboxylic acid degraders and methanogens, there will be an imbalance in the ecosystem. Carboxylic acids and H2 will accumulate and the pH of the system will fall, thus inhibiting methanogenesis. Waste stabilization is accomplished when methane gas and carbon dioxide are produced.

Equation 8 : CO2+ 4H2→ CH4+ 2H2O

Equation 9 : 2C2H5OH + CO2→ CH4+ 2CH3COOH

Equation 10 : CH3COOH → CH4+ CO2

The general scheme of anaerobic substrate biodegradation and microbial community relationships is illustrated in Figure 9.